The element manganese (Mn) may exist in six different valence (oxidation) states. Of particular interest and usefulness in the Pahlman™ systems and processes are those oxides of manganese having valence states of +2, +3, and +4, which correspond to the oxides MnO, Mn2O3, and MnO2. The oxide MnO4 is probably a solid-solution of both the +2 and +3 states.
A characteristic of most oxides of manganese species is non-stoichiometry; that is, most oxides of manganese molecules or MnO2 species typically contain on average less than the theoretical number of 2 oxygen atoms, with numbers more typically ranging between 1.5 to 2.0. The non-stoichiometry characteristic is thought to result from solid-solution mixtures of two or more oxide species, and exists in all but the beta (β), or pyrolusite, form of manganese dioxide. Oxides of manganese having the formula MnOX where X is about 1.5 to about 2.0 are particularly suitable for dry removal of target pollutants from gas streams. However, the most active types of oxides of manganese for use as a sorbent for target pollutant removal usually have the formula MnO1.7 to 1.95, which translates into manganese valence states of +3.4 to +3.9, as opposed to the theoretical +4.0 state. It is unusual for average valence states above about 3.9 to exist in most forms of oxides of manganese. The formula MnO2, as used herein, symbolically represents all varieties of manganese dioxide including those with valence states ranging from +3 to +4, or MnO1.5-2.0.
Oxides of manganese are known to exhibit several identifiable crystal structures, which result from different assembly combinations of their basic molecular structural units. These basic structural “building block” units are MnO6 octahedra, which consist of one manganese atom at the geometric center, and one oxygen atom at each of the six apex positions of an octahedral geometrical shape. The octahedra may be joined together along their edges and/or corners, to form “chain” patterns, with void spaces (“tunnels”). Regular (and sometimes irregular) three-dimensional patterns consist of layers of such “chains” and “tunnels” of joined octahedra. These crystalline geometries are identified by characteristic x-ray diffraction (XRD) patterns. Most oxides of manganese are classifiable into one or more of the six fundamental crystal structures, which are called alpha (α), beta (β), gamma (γ), delta (δ), epsilon (ε), and ramsdellite. Certain older literature also included rho (ρ) and lambda (λ) structures, which are now thought obsolete, due partly to improvements in XRD technique. Some (amorphous) forms of MnO2 exhibit no crystalline structure.
Certain characteristics of oxides of manganese probably arise from the size and shape of voids within these crystalline patterns and from certain elements, and compounds, which may occupy the voids and appear to help prevent collapse of certain structures. Applicants believe that these characteristics in addition to the oxidation state may have and affect upon the loading capacity of oxides of manganese sorbent. Further, many oxides of manganese are hydrous, and include structurally bound water. This bound water may also contribute to chemical reactivity and possibly catalytic behavior of the species.
Some oxides of manganese have the ability to absorb oxygen from gas. Manganous oxide (MnO) will oxidize to MnO2 in the presence of air, for example. Additionally, the dioxides are themselves oxidizers, they readily exchange oxygen in chemical reactions, and they are known to have catalytic properties. This oxygen exchange ability may be related to proton mobility and lattice defects common within most MnO2 crystal structures. The oxidizing potential of MnO2 is advantageously utilized in target pollutant removal in the Pahlman™ and other systems and processes. Target pollutants, such as NOx and SO2 gases, mercury (Hg) and other pollutants, require oxidation of the species prior to reaction with MnO2 sorbent to form reaction products, such as manganese sulfates and nitrates, mercury compounds, and other corresponding reaction products, in order for them to be captured and removed from gas stream.
Manganese compounds are soluble in water in the +2 valence state, but not in the +4 state. Therefore Mn+2 compounds, including MnO are readily soluble in aqueous solutions, as opposed to MnO2. During the formation of reaction products such as manganese nitrates and sulfates, the manganese is reduced from about the +4 state to the +2 state. This property allows the reaction products formed on the surface of oxides of manganese sorbent particles to be readily dissolved and removed from the sorbent particles in aqueous solutions by disassociation into sulfate, nitrate, and Mn+2 ions.
Manganese dioxides are divided into three origin-based categories, which are: 1) natural (mineral) manganese dioxide (NMD), 2) chemical manganese dioxide (CMD), and 3) electrolytic manganese dioxide (EMD). As implied, NMD occurs naturally as various minerals, which may be purified by mechanical or chemical means. The most common form of NMD is pyrolusite (β-MnO2), which is inexpensive, but has rather low chemical activity and therefore low pollutant loading capacity. CMD and EMD varieties are synthetic. EMD is produced primarily for the battery industry, which requires relatively high bulk density (which often results from relatively large, compact particles), relatively high purity, and good electrochemical activity. Though useful as a sorbent, characteristics such as low surface area and large compact particle size make EMD somewhat inferior to CMD for gas removal applications, despite its good electrochemical activity. Chemically synthesized oxides of manganese of all kinds falls into the CMD category and includes chemically treated oxides of manganese. In chemical synthesis, a great deal of control is possible over characteristics such as particle size and shape, porosity, composition, surface area, and bulk density in addition to electrochemical or oxidation potential. It is believed that these characteristic contribute the loading capacity of some oxides of manganese.
EnviroScrub Technologies Corporation has developed pollutant removal systems and processes utilizing dry and wet removal techniques and combinations thereof, incorporating the use of oxides of manganese as a sorbent. These systems and processes, commonly known as Pahlman™ systems and processes are the subject of co-pending U.S. patent application Ser. Nos. 09/919,600, 09/951,697, 10/044,089 and 10/025,270, the disclosures of which are incorporated herein. High target pollutant removal efficiencies have been obtained utilizing the Pahlman™ systems and processes with oxides of manganese as the sorbent. Due to pollutant loading during the removal process, the Pahlman™ system or some of its system components must be periodically taken off-line so that sorbent may be removed for regeneration and recovery of useful by-products. The frequency of such disruptions and related downtime could be decreased if it were possible to increase the loading capacity of the sorbent. It would therefore be desirable to enhance the loading capacities of the oxides of manganese in order to extend the period of sorbent use and its loading capacity. Applicants have developed methods of treating virgin oxides of manganese and of recycle processing of loaded oxides of manganese with oxidants or oxidizers that result in treated oxides of manganese having enhanced or increased loading capacity.